The present invention relates to a liquid-vapour distribution device for use in two-phase concurrent down-flow vessels. In particular, the invention concerns a liquid distribution device in form of a common header combining multiple vapour lift tubes (referred to as a BOXVLT) that improves distribution of liquid and vapour phases when a plurality of these BOXVLTs are assembled over a cross sectional area of a vessel in order to achieve thermal and compositional equilibrium in catalytic reactions. The device is in particular useful in hydroprocessing reactors.
The known designs of liquid distribution devices fall into one of five categories. The first is a series of troughs and overflow weirs to systematically subdivide the liquid into multiple streams before it contacts a catalytic bed. This type is often used in liquid contractors or countercurrent absorbers. An example of this type is described in U.S. Pat. No. 5,192,465.
A second type of liquid distribution devices is a perforated horizontal tray. This may or may not have notched weirs around the perforations. The tray may also have chimneys for vapour flow. This type of distribution device can be used for rough liquid distribution in conjunction with a more sophisticated final liquid distribution tray. Examples of this type are disclosed in U.S. Pat. No. 4,836,989.
The third common type of liquid distribution devices is a chimney tray. This device uses a number of standpipes laid out typically on a regular square or triangular pitch pattern on a horizontal tray. The stand pipes typically have holes in the sides for the passage of liquid. The tops of the standpipes are open to allow vapour flow down through the centre of the chimneys. Some designs use special vapour down-corner chimneys to handle the bulk of the vapour flow. This type is known from U.S. Pat. Nos. 4,126,540 and 3,353,924.
The fourth type of liquid distribution devices is a bubble cap tray. This device uses a number of bubble caps laid out on a regular-pitched pattern on a horizontal tray. The bubble cap is formed with a cap centred concentrically on a standpipe. The sides of the cap are slotted for vapour flow. Liquid flows under the cap and, together with the vapour, flows upward in the annular area and then down through the centre of the standpipe as described in U.S. Pat. No. 5,158,714.
The known trough type distribution device is mechanically complex and very sensitive to levelness. Depending on the design of the transitions between troughs, the quality of the distribution may also be susceptible to fouling.
The known perforated plate design is similar to the chimney design. The chimney design is preferred since it can be designed for a wider range of liquid/vapour loadings and is less susceptible to fouling.
A further known type of liquid distribution devices is perforated tray provided with vapour lift tubes in form of a long legged down-corner fitted with one or more short legged upcomers creating an up-flow zone and down-flow zone within the tube. The sides of the short legged upcomer are slotted and liquid flowing concurrently with vapour is lifted by means of the vapour flow upwardly in an up-flow zone and evenly distributed together with the vapour through the down-flow zone to an underlying catalyst bed as further described in U.S. Pat. No. 5,942,162.
The advantage of a vapour lift tube device over a chimney type design is the significantly wider turndown range possible with the vapour lift tube. As the liquid flow decreases, a properly designed chimney must either become taller or have smaller holes drilled in the side. Due to fabricating tolerances, care of installation and deflection due to operating load, not all of the distribution devices will be at the same level in the vessel. At some level of turndown, some holes will be covered with liquid and others will not. This results in uneven liquid distribution over the surface below the tray.